Neuromuscular system

Muscle fibers are activated by motor neurons, which transmit electrochemical signals from the spinal cord. Each motor neuron has multiple terminal branches, allowing it to innervate many muscle fibers. This innervation determines the muscle fiber type, characteristics, and role in exercise.

Activation of muscles

Muscle activation begins with impulses from motor neurons, which transmit signals from the spinal cord to the muscle fibers they innervate. A motor unit consists of a motor neuron and all the muscle fibers it controls. Activation follows the all-or-nothing principle, where all fibers within a motor unit contract together when stimulated.

The strength and precision of muscle movement depend on:

• Motor unit size: Small motor units are recruited for fine movements (e.g., in the eye), while large motor units are used for gross, powerful movements (e.g., in the quadriceps).
• Recruitment pattern: Force is increased by recruiting more motor units or by increasing the firing frequency of active motor units.

Motor unit recruitment follows the size principle, meaning smaller, low-threshold Type I fibers are activated first, followed by larger, high-threshold Type II fibers as force demands increase

Steps of muscle contraction

1. Energizing the myosin head: ATP is hydrolyzed by myosin ATPase, energizing the myosin head.
2. Cross-bridge formation: Myosin heads attach to binding sites on actin.
3. Power Stroke: Myosin pulls actin toward the center of the sarcomere, shortening the muscle. ADP is released.
4. Detachment: A new ATP molecule binds to myosin, detaching it from actin.
5. Reset: Myosin returns to its original position, ready to form another cross-bridge if calcium and ATP are available.

This cycle continues as long as calcium ions (released from the sarcoplasmic reticulum) remain bound to troponin, the calcium shifts tropomyosin and exposes actin’s binding sites. Calcium removal and ATP depletion result in muscle relaxation.

Muscle fiber types

Skeletal muscles are composed of two primary fiber types:

1. Type I (Slow-twitch):
   • High aerobic capacity and resistance to fatigue.
   • Used in activities like distance running.
2. Type II (Fast-twitch):
   • Subdivided into Type IIa and IIx fibers.
   • Generate rapid, powerful contractions but fatigue quickly.
   • Ideal for activities like sprinting and weightlifting.
   • With consistent training, Type IIx fibers can shift toward Type IIa characteristics.

Force production in muscle

Muscle force depends on the number of crossbridges formed between actin and myosin.

• Key factors:
  • Motor unit recruitment: Increasing the number of active motor units increases force.
  • Rate coding: Higher frequency of neural signals leads to summation and greater force output.
  • Motor unit synchronization: Coordinated firing of motor units enhances the efficiency and magnitude of force production.
  • Muscle cross-sectional area: Larger muscle size provides more contractile proteins, allowing greater force generation.

Characteristics of muscle fiber types

This table summarizes the main features of muscle fiber types and their functional properties:

This table summarize the relative Involvement of muscle fiber types in sport events:

Motor unit recruitment

Motor units are activated to increase force output by recruiting additional motor neurons.

• Key factors:
  • Small motor units (Type I fibers) are activated first for low-force tasks.
  • Larger motor units (Type II fibers) are recruited for high-force, high-speed tasks.
  • Effective training optimizes neural recruitment and coordination.

Proprioception

Muscle spindles

• Specialized sensory receptors located within muscles.
• Detect changes in muscle length and rate of stretch.
• Activation leads to reflex contraction to prevent overstretching.

Golgi tendon organs (GTOs)

• Located within tendons near the junction with muscle fibers.
• Monitor tension in the muscle-tendon unit.
• Activation inhibits further contraction to protect muscles from excessive force.

How can athletes improve force production

• Use heavy loads to enhance neural recruitment
• Increase muscle cross-sectional area
• Perform explosive multijoint exercises to maximize fast-twitch fiber activation
• Apply periodized training to systematically target both neural adaptations (coordination, recruitment, and rate coding) and hypertrophic adaptations (increased muscle size) across different training phases